§1Cold coil or hot coil
The great majority of springs — everything under about 9.5 mm of wire — are coiled cold. Beyond that, steel stops cooperating at room temperature.
Cold coiling stretches to 16 mm of bar when the spring index D/d is above about 6; tighter indexes or heavier bar demand hot coiling: the bar is brought to a distinct red heat — often above its hardening temperature — and wrapped while plastic. Four factors decide the route: bar size, spring index, the equipment on the floor, and the material itself. Hot-coiled springs are then hardened and tempered as formed parts, so the bars are bought annealed; sizes to 16 mm can alternatively be supplied oil-tempered for cold coiling. Finishes follow ASTM A331 practice — plain hot-rolled, cold-drawn, turned-and-polished, or centreless-ground where the surface must start clean.
§2Materials that must never be hot-coiled
Hot coiling assumes the material will be rebuilt by heat treatment afterwards. Any alloy that cannot be hardened by heating and quenching is ruined by the process.
Never hot-coil the copper-base alloys — spring brass, phosphor bronze, beryllium copper — nor Monel, Inconel, or the austenitic 18-8 / 300-series stainless steels. All of them take their strength from cold work (or age-hardening), which red heat erases without any quench-and-temper route to restore it. Their large-section problems are solved differently — see the stainless and non-ferrous sheets.
§3Carbon bars — ASTM A68 / SAE 1095
The cheapest and most widely used hot-coil material: plain high-carbon bar for railway, vehicle and lift-safety springs.
Round and square bars run to 50 mm and beyond, with rounded-edge rectangular sections to 150 mm wide and 25 mm thick for leaf work. The economics are unbeatable; the metallurgy has a known edge: hardenability is shallow, so heavy sections finish with a soft core, reduced shock resistance and more settling in service. Where an overload or a derailment-grade jolt is part of the duty, that is precisely the cue to move to the alloy bars of §4 — as buffer and safety springs for lifts, railway draft gear and truck suspensions did decades ago.
§4Alloy bars — the A331 families
Four alloy families cover hot-coiled practice, and one wartime substitute quietly became the first choice.
Chromium
Rounds and squares, annealed or oil-tempered — the long-serving automotive coil-spring steel (knee-action suspensions made its name). 5150 and 5155 trim the carbon slightly for formability.
Silicon-manganese
The classic railway substitute alloy, now rare domestically but long used in Britain under strict seam-control rolling. 9255 is the companion grade. High silicon lifts strength; surface quality is the thing to police.
Chromium-molybdenum
Railway and military equipment bars in a wide range of shaped sections — molybdenum for hardenability and hot strength where sections thicken.
Nickel-chromium-molybdenum
Introduced in WWII as a National Emergency substitute for chromium-vanadium — and it stayed, on merit: excellent surface, deep hardenability, notably clean of inclusions, and cheaper than Cr-V. The grades are near-interchangeable; 8660 is the recommendation above 25 mm, with bar available to 65 mm. Oil-tempered wire to 9.5 mm is an emerging supply form.
Alloy bars extend the temperature envelope well past cold-coiled wire: continuous service to about 427 °C and intermittent excursions to 510 °C are established practice.
§5Tool steels for springs
Occasionally a special spring is coiled from tool steel — and just as often it breaks early, because it was treated like a punch instead of a spring.
Warehouse drill rod — oil-hardening, 0.95–1.10 % carbon, precision-ground, in short 1 m and 4 m lengths — is the usual stock. The classic failure is hardness: tool-room habit runs HRC 58–62, which in a spring is simply a fracture waiting for its first full deflection. Springs want HRC 50–54. Harden the oil-hardening grades at 771–788 °C (5-minute soak), quench in oil until the section reaches bath temperature, and temper immediately at about 343 °C for 30–60 minutes.
High-speed steel earns its cost only at temperature: 18-4-1 (T1) springs have run continuously near 413 °C at torsional stresses to about 483 MPa. The treatment is pure tool-room ritual — preheat 843–871 °C, superheat 1282–1310 °C, oil-quench to about 93 °C, and temper at once at 677 °C for one to two hours to the same HRC 50–54 target. For allowable stresses, project the chromium-silicon A401 curves to the larger diameters; the tool steels sit at or above them.
§6The process schedule
One thermal discipline runs through all the bar steels: coil hot, cool, re-harden, quench warm, and temper straight from the quench.
| Step | Carbon · A68/1095 | Ni-Cr-Mo · 8645–8660 | Notes |
|---|---|---|---|
| Hot coil | 871 | 899 | coil at red heat, air-cool the formed spring |
| Re-harden | 843 | 871 | soak ≈20 min under 16 mm section, up to 40 min heavier |
| Quench | warm, moderately agitated oil, ≤66 °C | never water — deep-hardening chemistry cracks | |
| Transfer | remove while still 93–149 °C | temper immediately, never from cold | |
| Temper | 454–510 for ¾–1½ h | time by section and target hardness | |
The transfer row is the one that saves springs: a heavy section left to cool fully after quenching carries its transformation stresses unrelieved, and hot-coiled bars crack for exactly the same reason chromium-silicon wire does. Warm from the oil, straight to the temper furnace.
§7Design properties of hot-rolled bars
Hot-rolled surfaces carry a little decarburisation and roughness, and the design constants are deliberately trimmed to admit it.
| Property | Carbon bars | Alloy bars |
|---|---|---|
| E, tension | 199.9 GPa — reduced for surface condition | |
| G, torsion | 74.1 GPa — reduced for surface condition | |
| Tensile after H&T | 1207–1344 MPa | 1241–1379 MPa (higher by arrangement) |
| Elastic limit, tension | 65–75 % | 78–85 % |
| Elastic limit, torsion | 50–60 % | 60–70 % |
| Hardness, HRC | 40–44 | 45–50 |
| Density | 7.85 g/cm³ (0.284 lb/in³) | |
Compare these moduli with the cold-coiled wires of Sheet 2 — G drops from 77.2 to 74.1 GPa — and remember the reduction is not pessimism but an allowance for the skin the rolling mill leaves behind. Diameter tolerances are a study of their own: ASTM A29 devotes 31 tables to them, varying by finish and size, and the drawing should cite the applicable one rather than restate it.
§8Quick reference
When to hot-coil
Bar over 9.5 mm (16 mm if D/d > 6) or index under 6. Buy annealed; coil at 871–899 °C; harden 843–871 °C; oil ≤66 °C; temper 454–510 °C from the quench heat.
Grade ladder
A68/1095 for economy → 5160 automotive → 9260 railway heritage → 4150/4161 military → 8645–8660 the modern default (8660 above 25 mm). Continuous 427 °C, intermittent 510 °C.
Exclusions & numbers
No hot coiling of copper-base, Monel, Inconel or 300-series stainless. Design with E 199.9 / G 74.1 GPa, HRC 40–44 carbon or 45–50 alloy, tool-steel springs at HRC 50–54 — never tool-room hardness.